ECE 576 – Power System Dynamics and Stability Prof. Tom Overbye Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign 1 Lecture 14: Governors

Announcements Read Chapters 4 and 6 Homework 4 is due March 10 Midterm exam is moved to March 31 (in class) because of the ECE 431 field trip on March 17 2

Speed and Voltage Control 3

Prime Movers and Governors Synchronous generator is used to convert mechanical energy from a rotating shaft into electrical energy The "prime mover" is what converts the orginal energy source into the mechanical energy in the rotating shaft Possible sources: 1) steam (nuclear, coal, combined cycle, solar thermal), 2) gas turbines, 3) water wheel (hydro turbines), 4) diesel/ gasoline, 5) wind (which we'll cover separately) The governor is used to control the speed Image source: 1/1e/Centrifugal_governor.png 4

Prime Movers and Governors In transient stability collectively the prime mover and the governor are called the "governor" As has been previously discussed, models need to be appropriate for the application In transient stability the response of the system for seconds to perhaps minutes is considered Long-term dynamics, such as those of the boiler and automatic generation control (AG), are usually not considered These dynamics would need to be considered in longer simulations (e.g. dispatcher training simulator (DTS) 5

Power Grid Disturbance Example 6 Time in Seconds Figures show the frequency change as a result of the sudden loss of a large amount of generation in the Southern WECC Frequency Contour

Frequency Response for Generation Loss In response to a rapid loss of generation, in the initial seconds the system frequency will decrease as energy stored in the rotating masses is transformed into electric energy – Solar PV has no inertia, and for most new wind turbines the inertia is not seen by the system Within seconds governors respond, increasing the power output of controllable generation – Many conventional units are operated so they only respond to over frequency situations – Solar PV and wind are usually operated in North America at maximum power so they have no reserves to contribute 7

Governor Response: Thermal Versus Hydro 8 Thermal units respond quickly, hydro ramps slowly (and goes down initially), wind and solar usually do not respond. And many units are set to not respond! Time in Seconds Normalized output

Some Good References Kundur, Power System Stability and Control, 1994 Wood, Wollenberg and Sheble, Power Generation, Operation and Control (2 nd edition, 1996, 3 rd in 2013) IEEE PES, "Dynamic Models for Turbine-Governors in Power System Studies," Jan 2013 "Dynamic Models for Fossil Fueled Steam Units in Power System Studies," IEEE Trans. Power Syst., May 1991, pp "Hydraulic Turbine and Turbine Control Models for System Dynamic Studies," IEEE Trans. Power Syst., Feb 1992, pp

2600 MW Loss Frequency Recovery 10 Frequency recovers in about ten minutes

Eastern Interconnect Frequency Response Can be quantified as MW per 0.1 Hz maximum deviation (a higher number is better) 11 uencyResponse.aspx

Frequency Response Definition FERC defines in RM13-11: “Frequency response is a measure of an Interconnection’s ability to stabilize frequency immediately following the sudden loss of generation or load, and is a critical component of the reliable operation of the Bulk-Power System, particularly during disturbances and recoveries.” Design Event for WECC is N-2 (Palo Verde Outage) not to result in UFLS (59.5 Hz in WECC) 12 Source: wecc.biz/Reliability/Frequency%20Response%20Analysis%20-%20Dmitry%20Kosterev.pdf

Frequency Response Measure 13 Source: wecc.biz/Reliability/Frequency%20Response%20Analysis%20-%20Dmitry%20Kosterev.pdf

WECC Interconnection Performance 14 Source: wecc.biz/Reliability/Frequency%20Response%20Analysis%20-%20Dmitry%20Kosterev.pdf

ERCOT Frequency Response As expected, smaller grids have greater frequency sensitivity 15 Source: M.G. Lauby, et. al., “Frequency Response Assessment of ERCOT and Québec Interconnections,” NAPS 2014 Graph shows MW per 0.1 Hz variation

Control of Generation Overview Goal is to maintain constant frequency with changing load If there is just a single generator, such with an emergency generator or isolated system, then an isochronous governor is used – Integrates frequency error to insure frequency goes back to the desired value – Cannot be used with interconnected systems because of "hunting" 16 Image source: Wood/Wollenberg, 2 nd edition

Generator “Hunting” Control system “hunting” is oscillation around an equilibrium point Trying to interconnect multiple isochronous generators will cause hunting because the frequency setpoints of the two generators are never exactly equal One will be accumulating a frequency error trying to speed up the system, whereas the other will be trying to slow it down The generators will NOT share the power load proportionally. 17

Isochronous Gen Example WSCC 9 bus from before, gen 3 dropping (85 MW) – No infinite bus, gen 1 is modeled with an isochronous generator (PW ISOGov1 model) 18 Gen 2 is modeled with no governor, so its mechanical power stays fixed

Isochronous Gen Example Graph shows the change in the mechanical output 19 All the change in MWs due to the loss of gen 3 is being picked up by gen 1

Droop Control To allow power sharing between generators the solution is to use what is known as droop control, in which the desired set point frequency is dependent upon the generator’s output 20 R is known as the regulation constant or droop; a typical value is 4 or 5%. At 60 Hz and a 5% droop, each 0.1 Hz change would change the output by 0.1/(60*0.05)= 3.33%

WSCC 9 Bus Droop Example Assume the previous gen 3 drop contingency (85 MW), and that gens 1 and 2 have ratings of 500 and 250 MVA respectively and governors with a 5% droop. What is the final frequency (assuming no change in load)? 21

WSCC 9 Bus Droop Example The below graphs compare the mechanical power and generator speed; note the steady-state values match the calculated Hz value 22 Case is wscc_9bus_TGOV1

Quick Interconnect Calculation When studying a system with many generators, each with the same (or close) droop, then the final frequency deviation is The online generator summation should only include generators that actually have governors that can respond, and does not take into account generators hitting their limits 23 The online generators obviously does not include the contingency generator(s)

Larger System Example As an example, consider the 37 bus, nine generator example from earlier; assume one generator with 42 MW is opened. The total MVA of the remaining generators is With R= Case is Bus37_TGOV1